The application generally relates to a method of controlling a chiller system using a variable speed drive (VSD). The application relates more specifically to the use of a variable speed drive (VSD) to maintain cooling capacity during short electrical power interruptions, more commonly known as “ride through” or “coast through”.
Many cooling applications and processes rely upon a continuous flow of coolant or cooling capacity. Interrupting a process or production line cooling application due to a momentary power failure may result in significant product loss and lengthy recovery downtime, resulting in great expense and loss of revenue. Preventing such a loss is of significant value.
Short duration power interruptions can damage rotating equipment if power is restored while the compressor motor is coasting down. Damage can occur if the applied power is out of phase with the power generated by the coasting motor due to negative sequence torques that can break motor shafts and compromise motor electrical insulation. In some existing chiller systems the VSD or chiller control panel can detect a power loss within a fraction of a line cycle and quickly shut down the chiller to prevent damage.
Currently during a power interruption, the motor starter is disengaged, electrical power is inhibited from the chiller, and the chiller mechanical driveline is stopped until the fault clears. Typically restarting the cooling system includes a safe coast down cycle, lubrication cycle, and restart cycle, which add two to four minutes to the cooling system restart process, during which there is no cooling capacity delivered to the cooling load. Due to this pause in the chiller system restarting process, in a production process a thermal build-up of heat must be removed before the production process can return to capacity. Quick restart capabilities may be employed to shorten the effects of a power interruption, however; the chiller must come to a complete stop before restart can begin.
Semiconductor Equipment and Materials International (SEMI) Standard F47-0706 Specification for Semiconductor Processing Equipment Voltage Sag Immunity sets requirements and recommendations for voltage sag immunity for equipment used in semiconductor manufacturing. Chillers and adjustable speed drives are listed as a secondary focus of SEMI F47. In a ride-through situation with a drive (output power remains controlled), input voltage sag will result in an input current spike and torsional transient. To keep a drive active during larger voltage sags, the drive would need to be greatly oversized to prevent over current trips—an extremely expensive solution.
SEMI F47 Pass/fail criteria for Subsystems and Components, states: “May not perform at full rated operation but recovers operation without operator and/or host controller intervention.”
Therefore there is a need for a controller in a chiller system that can maintain a cooling load over an extended interval upon loss of electrical input power to the chiller system.
Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.